Effect of outer block interleaving in turbo codes performance over Rayleigh fading channels for 3GPP

Outer block interleaving improves the performance of turbo codes in correlated Rayleigh fading channels. The longer the outer interleaver, the better is the performance, with the penalty of more delay. Power could be saved by choosing the optimum interleaver for different operating environments. The four possible outer block interleaver lengths specified for 3GPP are examined for correlated Rayleigh fading channels with different normalized fade rates f_dT_s. SOVA and log-MAP algorithms are used in the turbo decoder, with a turbo interleaver length of 5114 bits. Our results show that choosing a TTI (time transmission interval) of 40 msec instead of 80 msec does not make a very large difference in performance, while the highest performance gain occurs for a TTI of 20 msec. We discuss the selection of the optimum interleaver length for the same bit-rate, but different scenarios.